This invention generally relates to an automatic powershifting transfer case providing two-wheel drive, all-wheel drive (AWD) and four-wheel drive (4-WD) modes. More particularly, the transfer case is hydraulically shifted and electronically controlled to shift between high- and low-speed ranges, two-wheel drive, all-wheel drive or part-time four-wheel drive modes.
Four-wheel drive capabilities are becoming more popular in both light trucks as well as passenger vehicles. It is generally the case that such a vehicle will provide either two- or four-wheel drive modes of operation, with the mode of operation being selected manually by the vehicle operator. The torque transfer devices used in such vehicles do not provide simple operation or effective implementation of various drive modes automatically. Such transfer cases have used planetary gear sets to selectively apply torque to output shafts for two-wheel or four-wheel drive modes. In four-wheel drive operation, the output shafts are differentially driven, with torque divided between the output shafts permitting speed variation between the shafts. Alternatively, the output shafts can be selectively interlocked to drive the shafts without slippage therebetween. Transfer cases of this type have conventionally required shift means manually operated within the driver's compartment to shift between the two-wheel and four-wheel drive modes.
Similarly, prior art vehicle transfer cases have also provided high and low speed drive ranges, with shifting means for shifting between high and low speed drive ranges. Various problems have been encountered with affecting such shifting "on-the-fly" during vehicle operation. Further, shifting of the transfer case between high and low ranges is generally performed by the vehicle operator manually from within the driver's compartment. Again, the transfer cases have not provided automatic control of shifting between high and low speed ranges.
On the other hand, operator control over the drive mode of the vehicle is a desirable feature under many circumstances, with the ability to manually shift between high speed and low speed drive ranges, as well as to shift a center-differential gear assembly between two-wheel, four-wheel differential-free, and four-wheel differential-locked drive modes.